This invention relates to fuel cells and more particularly to an array of microtubes forming an interconnected, 3-dimensional network that can be prepared by a direct-ink-writing (DIW) process.
As a consequence of ever-increasing energy demand and decreasing resources of fossil fuels, research activities have been focusing on new energy sources for many years. In electrochemical fuel cells, the chemical energy of a redox reaction is converted directly to electrical power, thus yielding a higher efficiency than conventional combustion processes. In addition, they have received much attention due to their versatility and low emission levels, which makes them promising candidates for clean, flexible, and—at the same time—highly economical power supplies.
Different approaches, ranging from low temperature devices with proton conducting polymer membranes to molten carbonate fuel cells (MCFC) and solid oxide fuel cells (SOFC) operating at high temperatures, have been proposed and introduced in the market [Sin1]. Compared to other concepts, the latter offers several particular advantages. Using a solid instead of a liquid electrolyte eliminates problems such as corrosion or containment. In contrast to polymer membrane-based devices, solid oxide electrolytes are not prone to CO poisoning, and no water management is needed to ensure their functionality [Bar1]. At least at an intermediate stage, their multi-fuel capability is also of interest [Blu1]. Whereas low-T devices use pure hydrogen, SOFC can be operated with various hydrocarbons, e.g, methane, natural gas or even CO without an internal reformer. To date, Yttria-stabilized ZrO2 (YSZ) is used as the solid electrolyte in SOFC. Ni-YSZ composite cermet (anode) and perovskite Sr-doped LaMnO3 or (La,Sr)(Co,Fe)O3 solid solutions (cathode) have been widely studied as standard electrodes for SOFC [Sin1], [Rot1]. The expressions in square brackets refer to the references included herewith. The contents of all of these references are incorporated herein by reference.
In spite of the numerous advantages of SOFCs, many challenges in device design remain, so that these systems are still far from replacing well-established energy sources. One major limitation is the need for high operating temperatures, due to the fact that dense electrolytes without pinholes have to be provided to separate the air and fuel chamber. To prepare reliable, dense membranes, a minimum electrolyte thickness is required. At the same time, however, the total internal resistance of the fuel cell device needs to be sufficiently small to ensure a high cell performance. Consequently, the specific conductivity of the solid electrolyte must exceed a specific threshold value. In the case of the traditional electrolyte material in SOFC, namely yttria stabilized zirconia (YSZ), this requirement leads to operating temperatures in the range of 9° C. to 1000° C. [Bra1]. This implies the need for expensive sealants and interconnects of single cells, both of which must withstand high temperatures, as well as requiring complex manufacturing methods [Las1]. Therefore, one major step towards promotion of SOFC power sources is cost reduction by achieving lower operating temperatures. For this purpose, novel electrolytes have been studied. As an alternative to YSZ, Gd-doped ceria (CGO) may be used at temperatures even below 600° C., while still presenting high ionic conductivity in combination with low electronic transfer numbers [Kha1], [Bra2]. In addition, alternative cell designs are of great interest.
Conventionally, fuel cells are designed in a two-chamber set-up with reduction of oxygen (cathode reaction, air side) and oxidation of fuel (anode reaction, fuel side) taking place in separated chambers on either side of the respective electrolyte. Up to now, two SOFC designs have been predominantly studied: tubular cell stacks (cf. FIG. 2a), which are of particular interest for stationary applications, and planar cells (cf. FIG. 2b), which are expected to serve in mobile applications due to their superior power density [Sin1] [Blu1]. Such a mobile system is of interest as an auxiliary power unit (APU) in automotive applications. In 2001, for example, BMW and Delphi introduced an APU based on SOFC technology for luxury cars.
With the tubular concept, which has been pursued for example by Siemens-Westinghouse for power generation systems [Sin1], high-temperature gas sealing is a minor issue. The single tubes, presenting an inner diameter of approximately 2 cm, a wall thickness of 2 mm, and an active length of up to 150 cm, are arranged in rows to create cell stacks. High-temperature stable interconnects assure electrical connectivity between the single cells as shown in FIG. 3.
Issues and limitations of the tubular device design include the following:                tubular stacks present a low volumetric efficiency due to the low surface-to-volume ratio        assembly of the cell stacks is difficult and thus leads to high cost        high-temperature stable interconnects have to be provided        they offer minimum potential for miniaturization or integration in MEMS devices        
An object of the present invention is a fuel cell design that overcomes the limitations of conventional tubular design, namely, its low volumetric efficiency, difficult assembly and high losses.